Communication systems known in the art use polarization diversity to improve system performance. For example, dual polarized base station antennas often include two ports that individually radiate or receive signals of orthogonal polarizations. These antennas typically are directional in azimuth and are used for sectoral coverage. Therefore, it is desirable for the two antenna ports to have equal azimuth beamwidths.
Known cellular base station installations are designed to provide 360 degree coverage divided into three 120 degree wide sectors. Dual polarized sector coverage base station antennas with both vertical and horizontal polarizations and nearly equal azimuth beamwidths of about 120 degrees are desirable. However, such antennas have been difficult to design. This is because a simple dipole can be appropriately placed over a small ground plane to achieve a 120 degree beamwidth in the H-plane, but not in the E-plane.
To overcome the known design difficulties of producing vertical and horizontal polarized radiation patterns with azimuth beamwidths of about 120 degrees, known antennas have employed dual slant polarizations (+/−45 degrees). Characteristics related to geometric symmetry in the antenna structure provide comparable beamwidths for each polarization.
However, the use of dual slant polarizations has been insufficient for several reasons. First, on mechanical boresight of a dual slant polarized antenna, the two polarizations are predominantly orthogonal. However, at angles off boresight, the polarizations become progressively less orthogonal until at 90 degrees azimuth, the polarizations are predominantly vertical. This characteristic results in a reduction of polarization diversity gain.
Furthermore, dual 45 degree slant antennas typically exhibit poor port-to-port isolation performance because the array elements of one polarization are not orthogonal to all elements of the other polarizations. This results in significant coupling between various elements of the two polarizations, thus degrading isolation.
In view of the above, there is a continuing, ongoing need for a structure that can provide a 120 degree E-plane half power beamwidth. Preferably, such a structure can be easily adjusted for other beamwidths and provide high isolation between polarizations.